In direct sequence spread spectrum digital wireless communications networks, a necessary part of the communications process for the wireless device is to find certain signals that are transmitted by base stations. An example of such a signal is the pilot channel. In order for the wireless device to find the pilot channel, the wireless device makes use of a searcher. The searcher attempts to acquire synchronization with the base station transmitted pseudo-random number (PN) sequence transmitted on the pilot channel. Synchronization can be used for a variety of purposes, including cell selection (for use in cell hand-offs), finger assignment and maintenance (for use in multipath combining to maximize received signal strength), channel profile estimation, location identification, and so forth.
Synchronization involves correlating a received signal (the PN sequence transmitted on the pilot channel, for example) with a locally stored version of the PN sequence with its PN offset adjusted. The PN offset of the PN sequence, along with other information, such as a dwell time, a window size, and so forth, is commonly referred to as a hypothesis. If the two PN sequences match, then the correlation results in a large value, while the correlation for two PN sequences that do not match results in a small value. The result of the correlation can then be accumulated (both coherently and non-coherently). The accumulated values can then be compared against a threshold. If the accumulated value exceeds the threshold, then the hypothesis can be declared to be a good hypothesis. Obtaining the PN offset is effectively, acquiring the pilot channel.
In direct sequence spread spectrum wireless communications systems using orthogonal spreading codes, as long as the orthogonality is maintained, there is no interference between the signals being transmitted. Examples of such wireless communications systems include IS-95 (an early CDMA (code-division multiple access) communications system standard), CDMA2000 (a third generation CDMA communications system standard), and UMTS (Universal Mobile Telephony System, also a third generation CDMA communications system standard). In these communications systems, the orthogonality of the signals being transmitted is maintained along the symbol boundaries. This means that if a correlation is taken along symbol boundaries, then the signals not being correlated will not interfere with the desired signal. However, if a correlation is taken in between symbol boundaries, then the orthogonality of the signals is no longer ensured and interference can take place.
Additionally, in direct sequence spread spectrum wireless communications using antenna diversity schemes to help improve performance, orthogonality is typically ensured only if accumulation is taken along slot boundaries. For example, if an accumulation straddles a slot boundary, then interference from other transmitted signals may take place and received signal strength can vary significantly.
A commonly used way to search for pilot channels (and other signals) typically involved the sample of received data at specific instances and the correlating the received data samples against different shifts (PN offsets) of the pilot channel PN sequence. This method is easy to implement and requires minimum buffering of the incoming data samples.
A solution to ensure that the accumulations take place along symbol boundaries, involves the hypotheses being tested being parsed and only those that take place along symbol boundaries will be tested. Hypotheses not resulting in symbol boundary accumulations can be held until such a time that their testing will occur along symbol boundaries.
One disadvantage of the prior art is that under very few instances is the accumulation aligned with the symbol boundaries. Therefore, interference from other signals being transmitted can take place and possibly reducing the signal-to-noise ratio of the communications system, hence its overall performance.
A second disadvantage of the prior art is that allowing only the hypotheses with symbol boundary aligned accumulations to be tested can severely reduce the overall throughput of the searcher. Hence the synchronization time can be very long, perhaps longer than what is permitted by the communications network.
A third disadvantage of the prior art is that forcing the hypotheses to wait until the proper time can require a significant amount of buffer space. Therefore increasing the cost and power consumption of the searcher and hence the wireless device.